Interpolating Polynomials?

[shackas]

Hi all,

I am way over my head here.

Basically I am trying to digitize aircraft performance tables in excel.

I have used webplotdigitzer and Plotly to extract the x,y values of the reference lines.

Right now I am only focusing on the far left side of the graph.
Two variables.
1 Temperature
2 Pressure altitude. Sea level up to 7000ft.

Because the lines are somewhat exponential I am using the formula A+B*Exp(C*X). Where X = Pressure altitude

Using Plotly, I have obtained values A , B , C for each of the reference lines. (7000,6000,4000,2000,0)

The trouble I am having is trying to calculate the correct y value on the graph when I input a pressure altitude.

For example,
A temperature of 20dg and a pressure altitude of 1900ft.
The x,y values should be x=30 y=12.12

However, I get a y value of 13.2248. I have used FORECAST.LINEAR, LINEST, TREND and GROWTH functions however they all output a y value that is not accurate.

Please can I have some assistance.

(There were graphs in the excel file, mini-sheet did not transfer them over.)

Thank you.

Perf Chart.xlsx
	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X
1
2
3
4
5
6			Example
7		OAT	30	0
8		ALT	30	12.12
9			52	12.12
10		Weight	60	10
11			94.5	10
12		Wind	99	9
13			110	9
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31		Marker				Temp			P.A
32		30	20			20	30		1900
33						0	20
34						slope	0.5
35						Temp	20
36						30
37
38
39
40								x	20		A + B*Exp(C*X)
41
42										A	B	C
43								for y	7000	-53.524	63.1887	0.00899
44									6000	-55.921	63.2525	0.00869
45									4000	-27.179	30.6974	0.01345
46									2000	-11.992	13.0894	0.0214
47									0	-93.202	89.8254	0.00404
48
49
50
51										7000	6000	4000	2000	0
52										29.2357	26.167	18.7786	12.8795	8.19117
53
54
55									Forecast.linear		13.2248
56
57
58									Linest		0.00307	7.39923
59
60
61									EXP LINEST		1.00307	1634.72
62
63
64
65
Sheet1

Cell Formulas
Range	Formula
G34	G34	=SLOPE(G32:G33,F32:F33)
F36	F36	=G35*G34+G33
J52	J52	=J43+K43*EXP(L43*$F$36)
K52	K52	=J44+K44*EXP(L44*$F$36)
L52	L52	=J45+K45*EXP(L45*$F$36)
M52	M52	=J46+K46*EXP(L46*$F$36)
N52	N52	=J47+K47*EXP(L47*$F$36)
K55	K55	=FORECAST.LINEAR(I32,J52:N52,J51:N51)
K58:L58	K58	=LINEST(J52:N52,J51:N51)
K61:L61	K61	=EXP(LINEST(J52:N52,J51:N51))
Dynamic array formulas.

 

[shackas]

Don't think the graph image uploaded

 

[shackas]

Image file is too large which is a bugger. If you can be bothered looking it up on google type "PA28 Takeoff Chart" into images.

 

[Fluff]

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